Amphipoda comprises about 6000 small, mostly marine, species. Most amphipods are laterally compressed in contrast with the depressed isopods. They have no carapace and have six pairs of abdominal appendages.
The largest and most familiar crustaceans belong to Decapoda. The 10,000 species of crabs, shrimps, crayfishes, lobsters, and their relatives are decapods. The first three segments of the decapod thorax are fused with the head to form a cephalothorax and their appendages are maxillipeds. The remaining five pairs of thoracic appendages bear simple or chelate walking legs.
Isopoda is a large and diverse taxon. Isopods are common in marine and freshwater habitats and there are important terrestrial and parasitic suborders. It includes the familiar terrestrial pill bugs, woodlice, and sea slaters, and about 4000 less familiar marine species.
Leptostracans, or phyllocaridans, are small, benthic marine crustaceans and there are about 20 species. Recent leptostracans are considered to be relics of a larger Cambrian taxon. They have an enormous bivalve carapace which encloses the thorax and most of the abdomen, and the compound eyes are stalked.
Stomatopods are predatory crustaceans that live in the shallow waters of tropical and subtropical seas and are known as the mantis shrimps. The carapace covers only the rear part of the head and the first three segments of the thorax. They are armed with a powerful pair of appendages topped with barbed tips, used to stab and snag prey and some have a blunt, calcified club on the elbow. They often attack prey larger than themselves.